This invention relates to a system for tracking a moving target, the said system itself being moving. It relates in particular to airborne radar systems, but also to systems on the ground or on board terrestrial or marine carriers.
Such systems include a locating and position measuring device in order to remain trained on the target. If, for example, radar is concerned, the locating and measuring device comprises a monopulse antenna associated with a transmitter and a receiver to supply range-measuring signals and signals representing angular divergence from the axis of the antenna. These signals are made use of by servo-controls which control the position of the antenna in such a way as to reduce error signals.
Each measurement which is made is subject to an error which is the resultant of errors of two kinds: statistical error and dynamic error. Statistical error, or measurement noise, can only be reduced by taking the mean of a number of successive measurements, which amounts to reducing the overall pass band of the system. However, processing of this kind results in a delay in servo-controlling the antenna, with the result that the later lags behind the event to be tracked, which thus gives rise to a dynamic error.
Reducing the resultant of statistical and dynamic errors can only result in a compromise between having a sufficiently narrow overall pass band for the system, which results in a small statistical error and a large dynamic error, and having a wider pass band, the effect of which is the reverse. This compromise is all the more difficult to put into practice in view of the fact that the solution adopted is not always applicable in every case. In effect, if the target is situated at long range, the measurement noise is low, and this permits a narrow pass band and reduced dynamic error. On the other hand, when range decreases, it is necessary to increase the pass band in order to reduce dynamic error, but this is done at the expense of measurement noise.
The accuracy of such systems is thus fairly limited, being of the order of 4 meters in respect of position and 10 meters per second in respect of speed for a target situated 1 kilometer away.